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Química Geral · Bonding and structure

Grupos funcionais, isômeros e primeiras setas curvas

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O que você vai aprender

  • Recognize the common functional groups, including the carbonyl families that differ only in what is attached to the C=O carbon.
  • Name simple alcohols, alkanes, aldehydes, ketones, alkenes and esters from the longest chain and the lowest locant for the principal group.
  • Calculate degrees of unsaturation from a molecular formula and use them to limit the possible structures.
  • Tell constitutional isomers, cis–trans isomers and enantiomers apart.
  • Identify nucleophiles and electrophiles and draw curved arrows for a proton transfer and a one-step substitution.

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Functional groups are the reactive parts

A functional group is a small arrangement of atoms that reacts in the same way whatever carbon skeleton carries it. The groups without C=O are recognized by their heteroatom: an alcohol has O–H on an sp³ carbon, a phenol has O–H directly on a benzene ring, an ether has C–O–C, an amine has nitrogen bonded only to carbon or hydrogen, and a haloalkane has C–F, C–Cl, C–Br or C–I. Alkenes (C=C), alkynes (C≡C) and arenes (benzene rings) are the carbon-only groups, and a nitrile is C≡N. The carbonyl families all contain C=O and differ only in what else is bonded to the carbonyl carbon: an aldehyde has at least one H, a ketone has two carbons, a carboxylic acid has OH, an ester has O–C, and an amide has N. So an OH or OR on the carbonyl carbon is part of an acid or ester, not a separate alcohol or ether, and a nitrogen on the carbonyl carbon makes an amide, not an amine. Paracetamol, drawn here, contains a phenol (the OH on the ring) and an amide (the NH bonded to C=O).

Naming simple molecules

Find the longest carbon chain that contains the principal functional group; its length gives the stem (meth-, eth-, prop-, but-, pent-, hex-). Number the chain from the end that gives the principal group the lowest locant, then add the suffix: -ol for an alcohol, -al for an aldehyde (always C1, so no locant), -one for a ketone, -oic acid for an acid, -ene for a C=C. When a molecule has two groups, the higher-priority one takes the suffix (acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine) and the other becomes a prefix such as hydroxy-. The molecule drawn is 5-hydroxyhexan-2-one: the ketone takes the suffix and the lowest number, and the OH is a hydroxy prefix. Esters are named as two words, the O-bonded alkyl group first and the acid part as -oate: CH₃COOCH₂CH₃ is ethyl ethanoate (ethyl acetate).

Degrees of unsaturation

Each ring or π bond removes two hydrogens from the saturated, acyclic formula CₙH₂ₙ₊₂. The degree of unsaturation (DBE) is therefore (2C + 2 + N − H − X)/2, where X counts halogens. Halogens count like hydrogen, each nitrogen adds one to the numerator, and oxygen and sulfur are ignored. Cyclohexene, C₆H₁₀, drawn here, has (12 + 2 − 10)/2 = 2: one ring and one C=C. A C=C or C=O counts 1, a C≡C or C≡N counts 2, and a benzene ring counts 4 (one ring and three π bonds). Hydrogenation over Pd/C at room temperature adds one H₂ to each C=C (two to a C≡C) but leaves ordinary rings and benzene rings intact, so for a non-aromatic hydrocarbon the H₂ uptake tells you how much of the DBE is π bonds.

Kinds of isomers

Isomers share a molecular formula but are different compounds. Constitutional isomers differ in which atoms are bonded to which: butan-1-ol and diethyl ether are both C₄H₁₀O. Stereoisomers have the same connections but a different arrangement in space. Rotation about a C=C bond does not occur at room temperature, so an alkene whose two alkene carbons each carry two different groups exists as cis and trans (Z and E) isomers, like the two but-2-enes drawn here. A carbon bearing four different groups is a stereocentre, and the molecule and its mirror image are non-superimposable enantiomers, as in butan-2-ol. Conformations, which interconvert by rotation about single bonds, are not isomers.

Nucleophiles, electrophiles and curved arrows

A nucleophile has electrons to give: a lone pair (hydroxide, water, ammonia, cyanide) or a π bond. An electrophile accepts electrons: an atom with an empty orbital (the boron of BF₃), a δ+ atom in a polar bond whose bonding pair can leave (the carbon of C–Br, the H of H–Cl), or a carbonyl carbon. A curved arrow shows a pair of electrons moving, so it always starts at a lone pair or a bond and points to the atom or bond that receives the pair. When hydroxide reacts with bromomethane, one arrow runs from an oxygen lone pair to carbon, forming C–O, and a second runs from the C–Br bond onto bromine, which leaves as bromide; without the second arrow carbon would have five bonds. A proton transfer uses the same two arrows: the base's lone pair to the H, and the old H–A bond onto A.

What to carry forward

Identify each group by what is bonded to its key atom, name molecules from the longest chain with the principal group at the lowest locant, use (2C + 2 + N − H − X)/2 to count rings and π bonds, decide whether isomers differ in connections or only in space, and draw every curved arrow from an electron pair to the electrophilic atom, breaking a bond whenever a new one would overfill an atom.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 4

Nucleophiles, electrophiles and curved arrows

  • A carboxylic acid gives its proton to an amineDraw the proton transfer from benzoic acid to methylamine that gives methylammonium benzoate, then the resonance arrows that share the carboxylate's negative charge between its two oxygens.
  • Methylation of 4-methylphenol through its phenoxideDraw the proton transfer that turns 4-methylphenol into its phenoxide, then the one-step substitution in which the phenoxide displaces iodide from iodomethane to give 1-methoxy-4-methylbenzene.
  • Hydrogen chloride gives its proton to waterDraw the two curved arrows for the proton transfer from hydrogen chloride to water that forms hydronium and chloride.
  • Hydroxide substitutes 1-bromopropaneDraw the two curved arrows for the one-step substitution of 1-bromopropane by hydroxide: a lone pair forms the C–O bond while the C–Br bond breaks, giving propan-1-ol.

Preveja o produto 1

  • Cyanide meets iodomethanePredict a substitution product by joining the nucleophile's lone-pair atom to the electrophilic carbon.

Classificar 16

Nucleophiles, electrophiles and curved arrows

  • Which species is a nucleophile?Identify a nucleophile as a species with an available lone pair.

Degrees of unsaturation

  • Which compound is not C₄H₈O?Use the degree of unsaturation to decide which structures can share a given formula.

Recognizing functional groups

  • Functional groups in aspirinIdentify the carboxylic acid and ester groups in aspirin by looking at what is bonded to each carbonyl carbon.
  • Functional groups in paracetamolRecognize a phenol and an amide in a drug molecule and avoid calling them an alcohol, amine or ketone.
  • Which compound is an amide?Distinguish an amide from a molecule that merely contains both an amine and a carbonyl group.
  • Which compound is an ester?Pick out an ester by finding an O–C group bonded directly to the carbonyl carbon.
  • Which compound is a phenol?Distinguish a phenol from an alcohol or ether that also contains a ring.
  • Functional groups in vanillinList every functional group of a multifunctional aromatic compound without merging neighbouring groups.

Isomers

  • The two but-2-enesClassify cis and trans alkenes as stereoisomers rather than conformations or constitutional isomers.
  • Butan-1-ol and diethyl etherClassify two compounds with the same formula and different connectivity as constitutional isomers.
  • Two drawings of butan-2-olRecognize two butan-2-ol drawings with opposite wedges at the only stereocentre as enantiomers.
  • Which alcohol exists as enantiomers?Find a stereocentre by checking for a carbon with four different groups.
  • Which alkene has cis and trans forms?Decide whether an alkene can exist as cis and trans isomers by checking both alkene carbons for two different groups.

Naming simple molecules

  • Name a branched alcoholName a branched alcohol by giving the OH-bearing carbon the lowest locant.
  • Name an esterName an ester by splitting it into the O-bonded alkyl group and the acid-derived -oate part.
  • Name a hydroxy ketoneApply functional-group priority and lowest locants to name a molecule with both a ketone and an alcohol.

Comparar 3

Nucleophiles, electrophiles and curved arrows

  • Rank C–X bond polarityRank carbon–heteroatom bonds by polarity from electronegativity differences.

Degrees of unsaturation

  • Rank by degree of unsaturationRank six-carbon hydrocarbons by degree of unsaturation from their structures.

Isomers

  • Which pair are not isomers?Check molecular formulas to decide whether two structures are isomers, even when they look alike.

Calcular 7

Degrees of unsaturation

  • Degree of unsaturation of benzeneCalculate the degree of unsaturation of a hydrocarbon from its formula and match it to rings plus π bonds.
  • Degree of unsaturation of C₅H₈Calculate the degree of unsaturation of a hydrocarbon formula by comparing it with the saturated CₙH₂ₙ₊₂.
  • Degree of unsaturation of caffeineCalculate the degree of unsaturation of a molecule containing several nitrogens and oxygens and check it against the structure.
  • Degree of unsaturation with a halogenTreat a halogen like hydrogen when calculating the degree of unsaturation.
  • Degree of unsaturation with nitrogenInclude nitrogen correctly when calculating the degree of unsaturation.

Isomers

  • How many C₄H₁₀O alcohols?Enumerate the constitutional isomers of a formula that belong to one functional-group family.
  • How many pentanes?Count the constitutional isomers of a small alkane without double-counting the same skeleton.

Encontre o erro 3

Nucleophiles, electrophiles and curved arrows

  • Find the missing arrowDiagnose a substitution mechanism that forms a new bond to carbon without breaking the old one.

Recognizing functional groups

  • Find the error in a functional-group listDiagnose the error of splitting an ester into a ketone and an ether.

Naming simple molecules

  • Fix an alkane nameDiagnose a name that was built on a chain shorter than the longest chain.

Interprete as evidências 3

Degrees of unsaturation

  • Rings or π bonds?Combine the degree of unsaturation with hydrogen uptake to count rings and π bonds separately.

Naming simple molecules

  • Draw 4-methylpent-3-en-2-oneDecode a name with a ketone suffix, an alkene infix and a branch locant into the correct structure.
  • Draw pentan-2-oneTranslate the stem, suffix and locant of a ketone name into a structure.

Trabalhe de trás para frente 1

  • Which bromide gives propan-1-ol?Work backwards from a substitution product to the halide whose C–Br carbon becomes the C–OH carbon.

Explique por quê 2

Nucleophiles, electrophiles and curved arrows

  • Why bromomethane reacts but methane does notExplain substitution reactivity from bond polarity and leaving-group stability.

Recognizing functional groups

  • Why an ester is not a ketoneExplain that carbonyl families are defined by the atoms bonded to the carbonyl carbon.

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